Enhancing Combustion in a Dump Combustor Using Countercurrent Shear: Part 2 — Heat Release Rate Measurements and Geometry Effects

Research to advance our understanding of the countercurrent shear flow has been conducted, with particular emphasis on those characteristics of countercurrent shear that are beneficial for combustion applications. Studies carried out in a backward-facing step combustor burning prevaporized JP10-air mixtures, have examined the implementation of counterflow as a means to enhance turbulent burning velocities, with the overall objective of increasing volumetric heat release rates and thereby create a more compact combustion zone. The dump combustor is characterized by a nominally two-dimensional primary flow mixture of prevaporized fuel and air, entering a rectangular channel before encountering a 2:1 single-sided step expansion. Flow separation over the sudden expansion and the resulting recirculation set up a countercurrent shear layer downstream of the dump plane and a low velocity zone conducive to flame anchoring. Combustion control strategies aim to increase turbulent kinetic energy and flame three-dimensionality in an effort to increase flame surface area and thus burning rates. A secondary flow is created via suction at the dump plane as a fluidic control mechanism to enhance the naturally occurring countercurrent shear layer. Counterflow is shown to elevate turbulence levels and volumetric heat release rates downstream of the step in the base geometry while concomitantly reducing the scale of the recirculation zone[1]. Modifications to the rearward-facing step geometry are investigated using Particle Image Velocimetry (PIV) under isothermal flow conditions in an effort to extend the near field interaction between the recirculation zone and the incoming primary flow, thus exploiting the benefits of counterflow as seen in the base geometry. Using chemiluminescence, relative heat release rates are shown to increase by 90% with a counterflow level of 6% of the primary flow by mass in the base geometry, and a 150% increase with a counterflow level of 2.4% in the modified step geometry.

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Controlling Volumetric Heat Release Rates in a Dump Combustor Using Countercurrent Shear

AIAA Journal ◽

10.2514/1.24059 ◽

2007 ◽

Vol 45 (6) ◽

pp. 1317-1323 ◽

Cited By ~ 3

Author(s):

A. A. Behrens ◽

P. J. Strykowski

Keyword(s):

Heat Release ◽

Release Rates ◽

Dump Combustor ◽

Countercurrent Shear

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Velocity Field Response of a Forced Swirl Stabilized Premixed Flame

Volume 4A: Combustion, Fuels and Emissions ◽

10.1115/gt2017-63936 ◽

2017 ◽

Cited By ~ 1

Author(s):

Kiran Manoharan ◽

Travis Smith ◽

Benjamin Emerson ◽

Christopher M. Douglas ◽

Tim Lieuwen ◽

...

Keyword(s):

Heat Release ◽

Shear Layer ◽

Recirculation Zone ◽

Stokes Equations ◽

Quantitative Agreement ◽

Forced Response ◽

Response Analysis ◽

Annular Jet ◽

Experimental Response ◽

Prediction Approach

This study is motivated by the necessity to develop a low order prediction approach for unsteady heat release response characteristics in lean premixed gas turbine combustors. This in turn requires an accurate description of the coherent hydrodynamic oscillations induced in the combustor flow by acoustic forcing. Time resolved velocity and flame position fields are obtained using sPIV and OH-PLIF measurements on a single nozzle, swirl-stabilized, premixed, methane-air flame in a model “unwrapped” annular combustor rig. A natural acoustic oscillation in the rig at 115 Hz results in a coherent flow oscillation that is concentrated primarily within the shear layer between the annular jet flow and the central recirculation zone. A linear stability analysis performed about time averaged base flow fields shows that the flow does not have any self-excited hydrodynamic modes. We then compare predictions from a forced response analysis at a forcing frequency of 115 Hz, based on the linearized Navier-Stokes equations for this coherent response. Good qualitative agreement between linear forced response analysis predictions and experimental response results, is seen for the spatial variation of velocity oscillation amplitude fields, away from the burner centerline. Further, good quantitative agreement between predictions and the experimental response is seen for the phase speed of velocity oscillations along the shear layer between the annular jet and the central recirculation zone. This phase velocity is an important flow field characteristic that has a significant impact on the heat release response that results from these coherent velocity oscillations. Present methods for forced response analysis assume uniform forcing amplitude along the radial direction at the forcing location, as well as, open flows along the streamwise direction. Both these assumptions are not strictly true for the present burner which has a center body on its axis. This maybe the reason for somewhat poor qualitative and quantitative agreement between experiments and predictions at the centerline.

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On the Mechanisms Affecting Fluidic Vectoring Using Suction

Journal of Fluids Engineering ◽

10.1115/1.2375125 ◽

2006 ◽

Vol 129 (1) ◽

pp. 91-99 ◽

Cited By ~ 3

Author(s):

R. D. Gillgrist ◽

D. J. Forliti ◽

P. J. Strykowski

Keyword(s):

Shear Layer ◽

Secondary Flow ◽

Mass Flow ◽

Pressure Field ◽

Countercurrent Flow ◽

Primary Flow ◽

Countercurrent Shear ◽

Low Pressures ◽

Direction Opposite ◽

Subsonic Jet

Suction was applied asymmetrically to the exhaust of a rectangular subsonic jet creating a pressure field capable of vectoring the primary flow at angles up to 15deg. The suction simultaneously creates low pressures near the jet exhaust and conditions capable of drawing a secondary flow along the jet shear layer in the direction opposite to the primary jet. This countercurrent shear layer is affected both by the magnitude of the suction source as well as the proximity of an adjacent surface onto which the pressure forces act to achieve vectoring. This confined countercurrent flow gives rise to elevated turbulence levels in the jet shear layer as well as considerable increases in the gradients of the turbulent stresses. The turbulent stresses are responsible for producing a pressure field conducive for vectoring the jet at considerably reduced levels of secondary mass flow than would be possible in their absence.

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Investigation of Combustion Oscillations of Premixed Dump Combustor Using Time-Resolved Particle Image Velocimetry

ASME 2014 Gas Turbine India Conference ◽

10.1115/gtindia2014-8288 ◽

2014 ◽

Author(s):

Ramgopal Sampath ◽

Vikram Ramanan ◽

S. R. Chakravarthy

Keyword(s):

Heat Release ◽

Shear Layer ◽

Large Scale ◽

Velocity Fields ◽

Small Scale ◽

Pressure Amplitude ◽

Time Resolved ◽

Dump Combustor ◽

Large Scale Vortex ◽

Chemiluminescent Intensity

The present work deals with time-resolved investigation of the flow field during acoustic self-excitation by a lean premixed flame in a dump combustor with varying equivalence ratio at a constant air flow rate. Simultaneous measurements of pressure fluctuations, velocity fields using Time resolved Particle imaging velocimetry (TR-PIV) and CH* chemiluminescence were performed. The pressure, velocity and chemiluminescent intensity time traces were Fourier transformed to estimate the frequency and amplitudes. Conditions of maximum pressure amplitude correspond to the prevalence of intermittent bursts in pressure, velocity, and chemiluminescent intensity. Further, Proper orthogonal decomposition (POD) is applied to the chemiluminescent intensity and velocity fields. The POD mode shapes are able to capture the modes pertaining to both the acoustic and vortex mode of flame/flow oscillations. The burst oscillations are understood by examining the sequence of time-resolved velocity and chemiluminescent intensity during their growth and decay regimes. The growth of oscillations is promoted by the flame heat release fluctuations following the pattern of the large-scale vortex roll-up in the recirculation zone downstream of the dump plane, causing a tendency of acoustic excitation at the vortex mode. As the amplitude rises, the natural acoustic mode of the duct is simultaneously amplified, leading to small-scale vortices shed from the step corner at the acoustic time scale. These small-scale vortices adversely interact with the large-scale vortex controlling the heat release, resulting in its weakening and hence the decay of oscillations. This behavior was further observed in the spatially averaged vorticity along the shear layer. In addition to this, the time traces of the pressure and the velocity fluctuations at the shear layer and located half step height from the separation point were overlapped. The overlapped time traces showed a drift in the instantaneous phase during which the growth and decay of the oscillations were observed.

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Vectoring Thrust in Multiaxes Using Confined Shear Layers

Journal of Fluids Engineering ◽

10.1115/1.483220 ◽

1999 ◽

Vol 122 (1) ◽

pp. 3-13 ◽

Cited By ~ 24

Author(s):

F. S. Alvi ◽

P. J. Strykowski ◽

A. Krothapalli ◽

D. J. Forliti

Keyword(s):

Shear Layer ◽

Mass Flow ◽

Shear Layers ◽

Continuous Control ◽

Primary Flow ◽

Flow Rates ◽

Thrust Vector ◽

Mass Flow Rates ◽

Countercurrent Shear ◽

Thrust Performance

A fluidic scheme is described which exploits a confined countercurrent shear layer to achieve multiaxis thrust vector response of supersonic jets in the absence of moving parts. Proportional and continuous control of jet deflection is demonstrated at Mach numbers up to 2, for pitch vectoring in rectangular nozzles and multiaxis vectoring in axisymmetric nozzles. Secondary mass flow rates less than approximately 2% of the primary flow are used to achieve thrust vector angles exceeding 15 degrees. Jet slew rates up to 180 degrees per second are shown, and the fluidic scheme is examined in both static and wind-on configurations. Thrust performance is studied for external coflow velocities between Mach 0.3 and 0.7. [S0098-2202(00)02601-8]

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Controlling Turbulence in a Rearward-Facing Step Combustor Using Countercurrent Shear

Journal of Fluids Engineering ◽

10.1115/1.1899170 ◽

2005 ◽

Vol 127 (3) ◽

pp. 438-448 ◽

Cited By ~ 21

Author(s):

David J. Forliti ◽

Paul J. Strykowski

Keyword(s):

Kinetic Energy ◽

Shear Flow ◽

Shear Layer ◽

Turbulent Kinetic Energy ◽

Near Field ◽

Sudden Expansion ◽

Turbulent Fluctuation ◽

Separated Shear Layer ◽

Field Control ◽

Countercurrent Shear

The present work describes the application of countercurrent shear flow control to the nonreacting flow in a novel step combustor. The countercurrent shear control employs a suction based approach, which induces counterflow through a gap at the sudden expansion plane. Peak turbulent fluctuation levels, cross-stream averaged turbulent kinetic energy, and cross-stream momentum diffusion increased with applied suction. The control downstream of the step operates via two mechanisms: enhanced global recirculation and near field control of the separated shear layer. The use of counterflow also enhances three dimensionality, a feature that is expected to be beneficial under burning conditions.

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Three-dimensional vortex dynamics in a rectangular sudden expansion

Journal of Fluid Mechanics ◽

10.1017/s0022112095001212 ◽

1995 ◽

Vol 289 ◽

pp. 1-27 ◽

Cited By ~ 21

Author(s):

J. R. Hertzberg ◽

C. M. Ho

Keyword(s):

Shear Layer ◽

Rectangular Channel ◽

Three Dimensional ◽

Sudden Expansion ◽

Vortex Rings ◽

Velocity Measurements ◽

Inlet Channel ◽

System Flow ◽

Component Velocity ◽

System Geometry

A detailed experimental study of flow in a rectangular sudden expansion using both active and passive forcing techniques has been made. The configuration consists of a 2:1-aspect-ratio rectangular channel which undergoes a sudden expansion such that the backward-facing step height (h) is uniform, and equal to the minor side of the inlet channel. Passive forcing was provided by the system geometry; the rectangular vortex rings formed in the shear layer of the expanding jet undergo self-induction, deforming the jet cross-section and introducing transverse velocities not found in plane or axisymmetric configurations. Active forcing was induced by periodic fluctuations in the system flow rate at the jet natural frequency. This served to enhance the unusual three-dimensional effects and phase-lock the flow for ensemble analysis. The results presented here include a description of the evolution of an isolated vortex in this configuration obtained from flow visualization of a suddenly started jet, as well as forced steady-state three-component velocity measurements which are used to characterize the flow field. The evolution of a rectangular vortex ring in the jet shear layer is traced, and both fluctuating and time-constant transverse velocities are related to the passage of the vortex structures.

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